Slushies vs. Frozen Underwear for Hot-Weather Workouts

Slushies or frozen underwear — which is better for staying comfortable while exercising in the heat? That pressing question motivated a new study comparing different ways of dealing with workouts in summertime temperatures. It concludes that precooling with ice is helpful, but the benefits depend on where you put the ice.

Exercising in the heat is more draining than most of us may remember, given this year’s long, chilly winter and spring in most of the country. When ambient temperatures rise, the body must deal with high internal and skin temperatures during exercise.

In response, the body sweats more, which allows for evaporative cooling from the skin, and also shunts blood away from the core and toward the skin to release additional heat.

These processes, while effective at removing heat from the body, increase strain on the heart and reduce the flow of blood and oxygen to working muscles, which can make a workout that was easy when the weather was cool feel grueling.

So, many athletes and scientists have begun experimenting with a technique called precooling that involves lowering the body’s temperature before a workout and, presumably, allowing someone to exercise more comfortably afterward, since the body starts from a lower baseline.

You can precool in many ways, with frigid showers, a cooling vest or shorts, an icy drink, and so on. Some of these techniques lower skin temperatures. Others — primarily those that involve ingesting ice — drop the temperature of your core.

Past studies have shown that, in broad terms, such precooling works and you can exercise in the heat for longer periods of time and at a higher intensity than if you do not first chill out.

But whether some types of precooling are more effective than others, and in particular whether cooling your skin would or would not be better than cooling your core, had remained unclear.

During each visit, the men ran strenuously for about 30 minutes on a treadmill while the scientists monitored their core and skin temperatures, heart rates, blood lactate levels, feelings about how hot they felt and other measures.

Before one of these runs, the men sat quietly in the heated lab for 20 minutes, sipping a room temperature, sweetened beverage.

Twenty minutes before another of the runs, they drank about 16 ounces of a sweetened slushy drink, which quickly and significantly lowered their core temperatures. (The researchers used slushies because sugar mixed with ice lowers the temperature of the resulting beverage more than if it were shaved ice alone.)

Finally, 20 minutes before a third run, the volunteers elaborately lowered their skin temperatures by draping cold, wet towels around their neck, sticking one arm into cold water, donning a frozen cooling vest, and slipping on underwear equipped with frozen ice packs at the thighs. Not surprisingly, their skin temperature fell considerably.

Then the men ran.

The results were interesting. Compared with when they did not precool, the men could sustain a higher relative intensity during the early part of their run after drinking the slushie or cooling their skin, and they reported feeling much less hot, especially after the skin cooling.

But by the end of each run, their core and skin temperatures were about the same, whether they had precooled or not, meaning that the benefits of precooling had been short-lived.

More surprising, while the men’s skin temperatures had climbed slowly after they had precooled with towels and vests and such, their core temperature had risen precipitously after the icy drink, faster even than with no precooling, suggesting that core cooling may wear off more rapidly than skin cooling, at least while running.

In the simplest terms, the upshot of the study, said Carl A. James, a doctoral candidate at the University of Brighton who led the experiment is that “if you can make people feel cooler, they generally run faster.”

But the results also indicate that if you use only one type of precooling, you might want to concentrate on icing your skin instead of your stomach, since the effects seem to linger longer.

On the other hand, for maximal effect, you could drink a slushie while wearing a chilled towel, vest and shorts and sticking your arm into a bowl of ice, Mr. James said, a labor-intensive chilling regimen that he hopes to study in the future.

Realistically, few of us will go such extremes. Instead, to feel and perform a bit better outside as temperatures rise, drape a frozen towel around your neck about 20 minutes before you start exercising or, if you prefer, have a slushie, Mr. James said.